In Python, not only can exceptions be handled, but they can also be triggered manually using the
raise statement. Raising exceptions is essential when you want to signal the occurrence of an anomalous event that should be addressed, either by the immediate code or by the calling code.
Raising exceptions can serve multiple purposes:
The basic syntax of
raise is straightforward. You can raise a built-in exception or a custom one.
if some_condition: raise Exception("This is an error message.")
Python has several built-in exceptions, such as
RuntimeError. You can raise these with custom messages that provide insight into the problem.
def set_age(age): if age < 0: raise ValueError("Age cannot be negative!") # ... rest of the code
You might sometimes want to catch exceptions, perform some actions (like logging), and then reraise the same exception. This can be done using
raise without an argument in an
try: # Some operation that can raise an exception x = 1/0 except ZeroDivisionError: print("Logging the error...") raise # reraises the caught exception
For more specific error handling, you can create your own exception classes. Custom exceptions should typically derive from the
Exception class or a derived class.
class NegativeAgeError(Exception): """Raised when age is negative.""" def set_age(age): if age < 0: raise NegativeAgeError("Age cannot be negative!")
raise ... fromSyntax
In Python 3, if you’re raising a new exception in response to another exception (for instance, translating exceptions), you might want to retain the original exception’s context. The
raise ... from syntax allows this.
try: x = int("not a number") except ValueError as e: raise TypeError("A number was expected!") from e
In this case, the
TypeError will indicate that it was directly caused by the
Raising exceptions is a powerful tool in the Python programmer’s toolkit. It allows you to enforce constraints, signal errors, and ensure that problematic states are caught and appropriately addressed. By understanding how and when to raise exceptions, you can make your programs more robust and maintainable.